Just make sure the switch costs at least as much as the preamp, and has rhodium-plated contacts as a bare minimum!! Don't forget to have it made from hepta-laminated (lucky 7's) silver, copper and gold of only the purest (5-nines minimum (99.999% pure)), most oxygen-free metal possible, and DON'T USE LEADED SOLDER TO MAKE THE CONNECTIONS, use the same grade silver as above, cast in place, to attach to the superconductor wires (what, don't tell me you don't have a supply of liquid nitrogen around!).

If I remember right, the FP circuit has a 470K resistor to ground after the coupling cap.

I'm not sure how the first schematic would work. If he switches at the input, then won't he end up with the amp's input impedance and the FP's output impedance in parallel? So his HT source will be driving a much lower impedance. Or is that not how it would work?

The other option is to use one of the inputs and add padding resistors to it so you end up with unity gain, or close to it. That way the FP will still be in the circuit, but it won't affect the volume of the front L and R channels (or at least, not too much). Of course, if you don't want the FP's sound in the picture, then you do need a bypass, that depends upon how picky you are about your HT sound.

Edit: I don't know if padding resistors would work, but I'm sure there are ways to make one of the inputs be "unity gain".